Power semiconductor products are often fabricated using N or P channel lateral double-diffused metal-oxide-semiconductor (LDMOS) transistors for high power switching applications. For instance, solenoid driver integrated circuits and other output drivers often include one or more such LDMOS power transistor devices along with logic and other lower power analog circuitry, wherein the LDMOS transistors are used to provide control outputs to solenoids in automotive or other applications. LDMOS devices advantageously combine short-channel operation with low on-resistance (Rdson) for high current handling capabilities and are able to withstand large blocking voltages. These devices are thus ideally suited for power switching applications, particularly where inductive loads are to be driven, and have been widely used for integrated circuit output drivers requiring blocking voltages in the range of 20-60 volts, and current carrying capability in the range of about 1-3 amps or higher. In addition, LDMOS device fabrication is relatively easy to integrate into CMOS process flows, facilitating use in devices where logic, low power analog, or other circuitry is also to be fabricated in a single IC.
For power transistors, a given device design is typically a tradeoff between breakdown voltage, on-state resistance, and safe operating area. Breakdown voltage is often measured as drain-to-source breakdown voltage with the gate and source shorted together (BVdss). Where high breakdown voltage is needed, drain-extended MOS transistors are often employed, in which the drain region is spaced from the gate to provide a drift region or drain extension in the semiconductor material therebetween. The spacing of the drain and the gate spreads out the electric fields thereby increasing the breakdown voltage rating of the device. However, the drain extension increases the on-state resistance of the drain-to-source current path Rdson. In conventional drain-extended MOS devices, the Rdson and breakdown voltage are thus generally inversely related, wherein the drain extension causes an increase in Rdson, thus limiting the drive current rating of the device.
The safe operating area (SOA) of a power transistor is a measure of the device's ability to turn off current to a reactive load, such as-a solenoid, without thermal failure or degradation. For a larger device SOA, a larger inductive current can be turned off without damaging the device. For instance, when attempting to turn off drive current to an inductive load such as a solenoid, the voltage across the device is greatly increased while the inductive load prevents the current from decreasing to zero instantaneously. For providing power to capacitive loads, the current through the device is greatly increased while the load prevents the device voltage from decreasing to zero instantaneously. Thermal SOA is a measure of the device size or area needed to accommodate thermal pulses during switching of inductive or capacitive loads, wherein devices that do not satisfy the minimum thermal SOA may suffer thermal degradation or failure during operation.
An ongoing trend in the semiconductor industry involves scaling or reducing the size of semiconductor device features, as product packages are becoming smaller and/or where more circuitry is added to existing package sizes. Such scaling efforts present challenges to transistor device designs, particularly where the design specifications for breakdown voltage, current carrying capability, and/or safe operating area remain the same. Conventional high power device layout techniques separate high and low power circuitry into different locations of a device die, wherein the high power devices are often located near the peripheral die edges to facilitate wiring to external pads. However, such device segregation layout techniques present a limitation on efforts to scale new product designs while still providing the required SOA and Rdson performance. Thus, there remains a need for improved transistor devices to facilitate product scaling without sacrificing SOA and Rdson performance.